Apparatuses of this kind are known, in particular, as sliding, rotary, or rotating disc microtomes depending on the direction of the relative motion, or as ultramicrotomes depending on the cut thickness. To produce the section, either the specimen is measurably shifted with respect to the stationary blade edge, or the blade edge is measurably shifted with respect to the stationary specimen. The thickness of the section is adjusted by measurably stepwise displacement of the blade edge or of the specimen holder perpendicularly to the current sectioning plane.
The approaching motion between the blade edge and specimen is referred to as an “advance.” The portion of the sectioning stroke during which the specimen is located in the engagement region of the blade edge is referred to as a “sectioning window.” To ensure good sectioning quality, the sectioning speed in the region of the sectioning window must not be too high. In the advance region, on the other hand, the positioning speeds can be as high as possible so as to minimize the total processing time.
Known apparatuses are therefore usually equipped with programmable alternating drives that permit the setting of different advance speeds and sectioning speeds. Once a blade edge and a specimen to be processed have been placed into the apparatus, the travel lengths for advancing and for sectioning the specimen should be capable of being detected and set by said apparatus in as automatic a fashion as possible.
It is known from DE 102 58 553 B4 (corresponding to US 2004/0124378) to arrange for this purpose, between the blade edge and specimen holder, a light barrier that contains a transmitter for producing a thin light beam bundle and a detector for receiving the light beam bundle. The light beam bundle is arranged parallel to the blade edge and spaced away therefrom at the same height. As long as the relative position between the specimen and blade edge is not known, a relative motion between the specimen and blade edge in a plane parallel to the sectioning plane must also be performed during advance, in order to ensure that specimens of different sizes arrive in the region of the light barrier.
A coding unit associated with the alternating drive ascertains, from the known distance between the light barrier and blade edge, the advance motion that is still possible, and switches over to the sectioning speed. The duration of the interruption of the light barrier during sectioning is proportional to the size of the sectioning window, and can likewise be ascertained by the coding unit and used for the next sectioning operation.
The disadvantage of the known apparatus is that a plurality of stepwise advances, combined in each case with a sectioning stroke, must be carried out in order to detect the light barrier. This requires a relatively large expenditure of time.